Increased attention has been focused on fuel economy (FE) of passenger vehicle lubricants. For diesel engine oils, FE is significantly lost in service due to soot loading in oils which inhibits adsorption of friction modifier (FM) on rubbing metal surfaces.On the other hand, soot may have a positive aspect because this has a similar molecular structure to graphite which may have a possibility to work well as a solid lubricant, i.e., FM in oils. This suggests that management of soot in diesel engine oils could achieve lower frictional performance, and thus enhance further improvement of fuel economy. The impact of soot properties on friction coefficient by physical and chemical characterization of soot accumulated in diesel engine oils have been firstly discussed in our previous paper.In this paper, the frictional response was investigated in the presence of soot at various test conditions and surface roughness. This paper also discussed the mechanism, i.e., how soot in oils could be acting as FM to reduce friction coefficient based on lab friction test data.